Rainforest may burn, speeding climate change

Posted: 17 February 2009

Without decisive action, global warming in the 21st century is likely to accelerate at a much faster pace and cause more environmental damage than predicted, according to a leading member of the Nobel Prize-winning Intergovernmental Panel on Climate Change.

IPCC scientist Chris Field of Stanford University and the Carnegie Institution for Science points to recent studies showing that, in a business-as-usual world, higher temperatures could ignite tropical forests and melt the Arctic tundra, releasing billions of tons of greenhouse gas that could raise global temperatures even more - a vicious cycle that could spiral out of control by the end of the century.

"There is a real risk that human-caused climate change will accelerate the release of carbon dioxide from forest and tundra ecosystems, which have been storing a lot of carbon for thousands of years," said Field, a professor of biology and of environmental Earth system science at Stanford, and a senior fellow at Stanford's Woods Institute for the Environment. "We don't want to cross a critical threshold where this massive release of carbon starts to run on autopilot."

Field presented his findings at the annual meeting of the American Association for the Advancement of Science (AAAS) in Chicago during a symposium titled, "What Is New and Surprising Since the IPCC Fourth Assessment?"

Field was a coordinating lead author of the IPCC fourth assessment, Climate Change 2007, which concluded that the Earth's temperature is likely to increase 2 to 11.5 degrees Fahrenheit (1.1 to 6.4 degrees Celsius) by 2100, depending on how many tons of greenhouse gases are released into the atmosphere in coming decades.

But recent climate studies suggest that the fourth assessment report underestimated the potential severity of global warming over the next 100 years. "We now have data showing that from 2000 to 2007, greenhouse gas emissions increased far more rapidly than we expected, primarily because developing countries, like China and India, saw a huge upsurge in electric power generation, almost all of it based on coal," Field said.

This trend is likely to continue, he added, if more developing countries turn to coal and other carbon-intensive fuels to meet their energy needs. "If we're going to continue re-carbonizing the energy system, we're going to have big CO2 emissions in the future," he said. "As a result, the impacts of climate change will probably be more serious and diverse than those described in the fourth assessment."

Forest-carbon feedback

Of particular concern is the impact of global warming on the tropics. "Tropical forests are essentially inflammable," Field said. "You couldn't get a fire to burn there if you tried. But if they dry out just a little bit, the result can be very large and destructive wildfires."

According to several recent climate models, loss of tropical forests to wildfires, deforestation and other causes could increase atmospheric CO2 concentrations from 10 to 100 parts per million by the end of the century. This would be a significant increase, given that the total concentration of CO2 in the atmosphere is currently about 380 parts per million, the highest in 650,000 years.

"It is increasingly clear that as you produce a warmer world, lots of forested areas that had been acting as carbon sinks could be converted to carbon sources," Field said. "Essentially we could see a forest-carbon feedback that acts like a foot on the accelerator pedal for atmospheric CO2. We don't exactly know how strong the feedback could be, but it's pretty clear that the warmer it gets, the more likely it is that degradation of tropical forests will increase the atmospheric CO2."

The ocean is another vital reservoir for carbon storage. Recent studies show that global warming has altered wind patterns in the Southern Ocean, which in turn has reduced the ocean's capacity to soak up excess atmospheric CO2. "As the Earth warms, it generates faster winds over the oceans surrounding Antarctica," Field explained. "These winds essentially blow the surface water out of the way, allowing water with higher concentrations of CO2 to rise to the surface. This higher-CO2 water is closer to CO2-saturated, so it takes up less carbon dioxide from the atmosphere."

Tundra thawing

Climate scientists also worry that permafrost in the Arctic tundra will thaw, releasing enormous amounts of CO2 and methane gas into the atmosphere. According to Field, the most critical, short-term concern is the release of CO2 from decaying organic matter that has been frozen for millennia. "The new estimate of the total amount of carbon that's frozen in permafrost soils is on the order of 1,000 billion tons," he said. "By comparison, the total amount of CO2 that's been released in fossil fuel combustion since the beginning of the Industrial Revolution is around 350 billion tons. So the amount of carbon that's stored in these frozen soils is truly vast."

Much of the carbon is locked up in frozen plants that were buried under very cold conditions and have remained in deep freeze for 25,000 to 50,000 years, he added. "We know that the Arctic is warming faster than anyplace else," he said. "And there is clear evidence that these frozen plants are very susceptible to decomposition when the tundra thaws. So melting of permafrost is poised to be an even stronger foot on the accelerator pedal of atmospheric CO2, with every increment of warming causing an increment of permafrost-melting that shoots an increment of CO2 into the atmosphere, which in turn increases warming.

"There's a vicious-cycle component to both the tundra-thawing and the tropical forest feedbacks, but the IPCC fourth assessment didn't consider either of them in detail. That's basically because they weren't well understood at the time."

Field's warning was echoed by French scientists, who said the IPCC's estimate that sea levels would rise around 40cm by 2100 was likely to be a best case scenario.

Former US vice-president Al Gore, who spoke at the meeting on Friday night, called for a globally coordinated stimulus to tackle climate change. "We've now reached the stage where continuing on our present course will threaten the entirety of human civilisation," he said.